In general, bases are desirably used in heat developable light-sensitive materials for accelerating heat development and in order to enhance stability of the light-sensitive material, bases must be used in the form of a precursor (which means in this specification a compound capable of being decomposed by heat to release a basic component). Bases practically suitable for such use must possess both stability at ordinary temperatures and rapid decomposability upon being heated.
As conventionally known base precursors, there are ureas as described in U.S. Pat. No. 2,732,299 and Belgian Pat. No. 625,554, urea or ammonium salts between urea and weak acids (Japanese Patent Publication No. 1699/65), hexamethylenetetramine or semicarbazides (U.S. Pat. No. 3,157,503), a combination of triazine compounds and carboxylic acids (U.S. Pat. No. 3,493,374), dicyandiamide derivatives (U.S. Pat. No. 3,271,155), N-sulfonyl-ureas (U.S. Pat. No. 3,420,665), amineimides (Research Disclosure, 1977, RD-15776), salts of thermally decomposable acids represented by trichloroacetic acid (British Pat. No. 998,949), etc.
However, image forming materials (using these compounds as base precursors have the essential defect that they fail to satisfy the aforementioned requirements of high stability at ordinary temperatures and rapid decomposability upon development processing. Therefore, images with a seriously reduced S/N ratio result due to low image density or due to release of the base during storage.